Recently released documents show that the British Secret Intelligence Service (MI6) used semen as an invisible ink! The technique came with its own particular challenges. According to the Telegraph, "at least one agent had to be reminded only to use fresh supplies of the 'ink' when correspondents began noticing an unusual smell."
I hearby offer my services to her Royal Majesty's Government. I'll eat as much melon as needed!
Thanks to "Friend of Colby" Bobby for passing this along. . .